1. Field of the Invention
This invention relates to an improved acrylic thermoplastic resin composition. More particularly, it relates to a sequentially produced multilayer acrylic thermoplastic resin composition which provides shaped articles having improved impact resistance, stress-whitening resistance, and solvent resistance.
2. Description of the Prior Art
Acrylic resin shaped articles are widely used both outdoors and indoors because they possess good transparency, weather resistance and thermoformability as well as an attractive appearance. However, acrylic resin shaped articles are not completely satisfactory in impact resistance and stress crack resistance. When they are exposed to certain solvents, they are subject to crazing or cracking. This is prominent particularly when there is internal and/or external stress. Thus, improvements in impact resistance and stress crack resistance have been eagerly sought after.
Many attempts have been heretofore made in order to enhance impact resistance and stress crack resistance. For example, it has been proposed to incorporate a rubber ingredient in a rigid acrylic thermoplastic resin in order to enhance the impact resistance. U.S. Pat. No. 3,793,402 teaches that the impact resistance can be enhanced without effect on the transparency by blending a rigid acrylic thermoplastic resin with a sequentially produced three- or more-stage polymer. U.S. Pat. No. 4,052,525 and Japanese Laid-open patent application No. 58,554/1978 teach an impact- and stress-whitening-resistant acrylic resin having an innermost hard layer, an elastomeric layer and an outermost hard layer and further having intermediate layers which interpose between the respective layers and have medial compositions between the adjacent layers. U.S. Pat. No. 4,180,529 teaches an impact- and stress-whitening-resistant acrylic resin having a first elastomeric layer, a second nonelastomeric layer, a third elastomeric layer and a fourth nonelastomeric layer.
Some attempts have also been made in order to improve the solvent resistance of acrylic resins. For example, Japanese Laid-open patent application No. 131,241/1974 teaches the incorporation of an alkyl acrylate or methacrylate oligomer in acrylic resins. Japanese Laid-open patent application Nos. 7,792/1978 and 99,190/1979 teach the copolymerization of methyl methacrylate with one or more certain acrylate or methacrylate monomers.
These attempts, however, have not been completely satisfactory because it is difficult to improve the impact resistance, the solvent resistance and the stress-whitening resistance to the desired extent, while maintaining the good transparency, weather resistance and thermoformability, inherent to acrylic resins.